RF energy transport through inductive coupling has been known for years and is used in, e.g., charging an electrical tooth brush. In its basic form, a loop is excited with a periodic (sinusoidal) current that creates a periodic magnetic field as indicated in FIG. 1. The receiver loop is placed in this periodically changing magnetic field and a periodic current is induced in this loop, so that a periodically changing voltage may be measured over a load connected to that loop, see FIG. 1.
Inductive coupling is very sensitive to loop (or coil, i.e., a multi-turn loop) positions. The loops need to be on top of each other and centred around the same axis, since the magnetic field lines will be concentrated around the transmitting loop axis.
By introducing capacitors at the loop terminals, a resonant system is created that is less position critical. RF power transfer takes place through non-radiative, resonant coupling, see FIG. 2. Resonant coupling is efficient (>95%) up to a coil-plane separation of about one coil diameter. Above that distance the power transfer efficiency drops quickly to zero. The coil circumference is much smaller than half a wavelength to prevent electromagnetic radiation. The resonance is obtained by the loop inductance in parallel with the capacitor. The usual frequency employed is 13.56 MHz.
With a loop circumference in the order of half a wavelength, the loop can become resonant by itself and start radiating electromagnetic waves. Since wavelength λ is related to frequency f as λ=c/f, where c is the velocity of light, a system can be obtained that at a low frequency acts as a non-radiative resonant coupling system and at a high frequency as an antenna-to-antenna coupling.
Unfortunately, the loop antenna created by increasing the frequency (loop circumference becomes about half a wavelength) has its maximum radiation and sensitivity in the plane of the loop, which means that the loops would have to be displaced sideways for a distant antenna-to-antenna coupling, see FIG. 3. However, it is desirable to have an antenna with its maximum radiation and sensitivity perpendicular to the loop plane, so that both loops/antennas can be separated in the perpendicular direction.
The powering and synchronization (data exchange) of Short Range Devices (SRD), especially smartphones, is traditionally performed by applying cables between the SRD and a power outlet and/or a computer. Recently, wireless powering through magnetic resonant coupling as described above has been introduced, which allows a contactless powering by placing the SRD on a platform containing transmitting coils. Within the SRD, a receiving coil is present. Both coils together form a magnetic resonance system through which energy is transformed at a low frequency.
For the powering, the SRD must be in close contact with the powering platform. For the synchronization, one has to resolve to cables or Bluetooth if possible. Charging of the SRD when this SRD is at some distance from the powering station is not possible.
A typical implementation as well known in the art is shown in FIG. 4. A planar array of loops is formed to operate as transmitter (1) in a magnetic resonance power transfer system. The receiver (8), i.e., the SRD, is equipped with a receiving loop. A subsection of the transmitting loops and the receiving loop form a magnetic resonance power transfer system.
Methods to activate the relevant transmission loops, i.e., finding the position of the SRD on the powering station, are known in the art. The magnetic resonance loops may be used simultaneously for data exchange by applying state of the art technology. As already mentioned, the power transfer efficiency can reach around 95% for transmitter and receiver coils separations within one receiver coil radius, dropping to zero over an additional radius of distance.
For larger separations, the transmitter and receiver loops are employed as resonant folded dipole antennas in a UHF or microwave, license-free, Industry Science and Medicine frequency band, e.g., 865.6-867.6 MHz and 2.446-2.454 GHz. The loops/antennas in the powering station are used as an array antenna. ‘Standard’ far-field RF power transfer is not feasible. The power density delivered at distances in the range 1-3 m from the base stations respecting the international transmit power limits would be too low to charge a typical SRD battery within a reasonable time.
This problem can be illustrated with the following example. A smartphone battery of 4V is assumed having a capacity of 1 Ah. The non-duty-cycled Effective Isotropic Radiated Power (EIRP) is limited in the 2.446-2.454 GHz frequency band to 0.5 W. The received RF power is given by the Friis transmission equation:
      P    R    =                    EIRP        ·                  G          R                    ⁢              λ        2                                      (                      4            ⁢            π                    )                2            ⁢              r        2            In this equation PR denotes the received RF power, GR the gain of the receive antenna, λ the wavelength used and r the separation between transmit and receive antenna. The gain of a folded dipole antenna is 1.64. This results in a received RF power at 2 m distance of 4.9 μW. Assuming further a perfect RF to DC conversion still means that charging a half-full battery would take 4e6/4.9 hours which is equivalent to about 93 years. If the frequency is lowered to 866 MHz, the received power at a distance of 2 m (transmitting a continuous EIRP of 3.28 W) will be 156 μW. Although the power density is better at the lower frequency, it is still too low for practical smartphone battery charging applications.
Hence, there is a need for a solution that increases the functionality of contemporary wireless charging systems.